1. Field of the Invention
The invention relates to radar video distribution and more particularly to a system that allows for the distribution of audio, video and computer data as well as radar data, via a digital distribution network.
2. Background Description
Traditional radar distribution has been implemented with a central analog radar switchboard which accepts inputs from multiple radars and provides outputs to a number of dedicated radar display consoles. The switchboard used a switch matrix to deliver one selected input radar video signal to each console. Such systems were dedicated to this purpose and generally were large and expensive. Recent developments in very high-speed digital networks make feasible the distribution of television video and audio over these digital networks, in addition to computer data. However, prior multiple radar systems coupled to multiple displays continue to use analog distribution networks because of the high bandwidth demand imposed on any digital systems implemented with prior techniques. A prior digital technique of digitizing the analog video data and transmitting it over FDDI rings could be used. However, this technique would fail to significantly reduce the bandwidth and, in addition, would require extensive data processing at each of the displays.
Prior radar systems were broadcast point-to-point without the unique pre-processing steps that are utilized in the system of the present invention. The present invention further reduces the bandwidth required to transmit digital radar video data to the display by pre-processing such data before the network receives it.
The present invention provides for the distribution of radar video to achieve a totally integrated radar distribution system over digital network technologies. The challenge for radar video distribution systems is to achieve maximum fidelity with minimum bandwidth. A typical shipboard radar distribution application is used to illustrate and quantify the problem:
1. Several radars are used (e.g., 5). PA0 2. Each radar has several video outputs (e.g., 2 to 4). PA0 3. Each video has a bandwidth up to 21 Mhz. PA0 4. Several display consoles (e.g., 20) may select any video source, each with a customized view of the data.
According to the Nyquist Theorem, a 21 Mhz signal must be sampled at a minimum of 42 Mhz to ensure reconstruction of the signal. If a digital resolution of 8 data bits per sample is assumed, the raw digitized video bandwidth requirement for a typical system may be calculated as follows:
5 radars * 3 videos (avg) *42 Mhz *8 bits=5.04 Gbps (*indicates multiplication)
This result is the bandwidth required to broadcast all videos at maximum resolution on the network simultaneously so that each console can extract from the network only the radar video it wishes to display. This bandwidth does not depend on the number of display consoles since each console can select one video from the complete set of videos and can perform the necessary processing to provide a unique view of the data. Typical high-speed networks offer bandwidths in the 100 Mbps to 10 Gbps range. The raw digitized video would severely burden even a high performance digital network, limiting the use of this network for other information, such as television and data.
Many data compression techniques exist that could reduce this bandwidth. These techniques fall into two categories, 1) lossless and 2) lossy. The lossy techniques have the disadvantage of reducing the fidelity of the video. With either technique, high-speed custom hardware is required to perform the compression and decompression functions at the rates required to handle real-time high-resolution radar video. Significant improvements have been achieved in recent years with compression algorithms but this has not generally addressed the unique characteristics of radar video.
Frame-based raster video compression algorithms (e.g., MPEG 2) may be used to distribute the scan converted raster image. This lossy approach requires compression and decompression hardware in addition to the scan conversion hardware, introduces additional display latency and fails to take advantage of the unique characteristics of the radar video in polar format. Alternatively, various lossless compression algorithms (e.g., run length encoding, arithmetic encoding, etc.) can be applied to the digitized radar video in polar format. Typically these can be expected to yield compression factors on the order of 10 times. This is not sufficient to reduce the network bandwidth required by radar to a desirable level. The use of lossy compression algorithms to achieve higher compression factors results in undesirable loss of fidelity. The compression problem may be complicated with new radars that randomly change maximum range and azimuth instead of simply rotating with a fixed maximum range.
Scan conversion is the process of converting radar data from a polar coordinate system (range and azimuth) to the Cartesian coordinate system (x,y) required for modern raster displays, typically in plan position indication (PPI) format. The scan converter also allows each console to select a unique view of the radar. The options for this view include range scale, (the range of radar data represented on the display), and offset, (the relative location of the radar origin on the display). Centered PPI is defined as a PPI display in which the radar origin is at the center, while offset PPI is defined as a PPI display in which the radar origin is not at the center of the display and may be off the display. While PPI is the most common radar display format, other scan conversion formats, such as range-height indication (RHI), B-Scan and A/R-Scan (amplitude-range) exist and may be implemented with the present invention.
U.S. Pat. No. 5,036,326 entitled "Method and Device for the Display of Several Radar Images in a Single Mosaic," issued Jul. 30, 1991 in the names of Jean-Pierre Andrieu, et al. shows a method and device for the display of several radar images in a single mosaic image. The present invention provides a network-based system that is also capable of providing mosaic images with enhanced flexibility, reduced cost and optimal display quality.